A monoazo lake pigment obtained by coupling an aromatic amine having a soluble group as a diazo component and .beta.-hydroxynaphthoic acid or .beta.-naphthol as a coupler component is widely used in the fields of printing inks, coating compositions and coloring of plastics. The monoazo lake pigment has been treated with a rosin for obtaining transparency and clearness in color tone and improving the dispersibility thereof. The rosin treatment is generally carried out by adding an alkali salt solution of a rosin (so-called "rosin soap") to a coupler component or a dye, and then adding a metal salt for a lake such as calcium chloride to precipitate the rosin on the surface of each pigment particle in the form of a rosin lake metal salt.
The above rosin treatment controls the pigment particles so as to have a fine particle size and improves the transparency and clearness of the pigment. Moreover, the rosin treatment has excellent effects that it as a coating prevents the coagulation of the pigment and that it improves the dispersibility of the pigment. However, it cannot be said that any conventional rosin treatment exhibits its full effects. In a pigment for an offset ink in particular, the amount of a rosin for improving the dispersibility is large. The use of the rosin in a large amount causes many problems such as a decrease in fluidity or a decrease in printing suitability caused by poor water resistance. Further, when the amount of the rosin is decreased, there are caused such problems as a decrease in transparency and clearness, an increase in bronze and a decrease in flushing compatibility.
It has been therefore desired to develop a surface treatment technique for effectively coating the pigment and to develop a monoazo lake pigment improved in flushing suitability, fluidity, clearness, gloss and freedom from bronze.
It is therefore an object of the present invention to provide an aqueous paste of a monoazo lake pigment, which can decrease the amount of the rosin so that the flushing time length can be decreased and that an exhausted water is less colored, and which is excellent in fluidity, clearness, gloss and freedom from bronze and also excellent in the suitability for offset printing, a process for the production thereof, and an offset ink.